a.) Don't include a zillion system headers from "extern.h" (actually, don't
include any).
b.) Break a global jmp_buf variable into a new file, so <setjmp.h> doesn't
have to be pulled in everywhere (it's only used in two places).
The (one line) new file is (c) TNF with a three-clause license, FW(L)IW.
This merges in all such remaining changes from the Linux port of the
NetBSD games, except in hunt (where substantial changes from OpenBSD
need to be looked at).
Some such changes were previously covered in PRs bin/6041, bin/6146,
bin/6148, bin/6150, bin/6151, bin/6580, bin/6660, bin/7993, bin/7994,
bin/8039, bin/8057 and bin/8093.
jwise